Non-destructive material evaluation refers to any of a wide variety of techniques that may be utilized to examine materials for defects and/or evaluate the materials without requiring that the materials first be destroyed. Such non-destructive material evaluation is advantageous in that all materials or products may be tested for defects. After being evaluated, acceptable (e.g., substantially defect-free or with acceptable defect levels) materials may be placed in service, while the defective materials may be re-worked or scrapped, as may be required. Non-destructive evaluation techniques are also advantageous in that materials already in service may be evaluated or examined in-situ, thereby allowing for the early identification of materials or components that may be subject to in-service failure. The ability to evaluate or examine new or in-service materials has made non-destructive material evaluation techniques of great importance in safety- or failure-sensitive technologies, such as, for example, in conventional aviation and space technologies, as well as in nuclear systems and in power generation systems.
One type of non-destructive evaluation technique, generally referred to as positron annihilation, is particularly promising in that it is theoretically capable of detecting fatigue and other types of damage in metals at its earliest stages. While several different positron annihilation techniques exist, as will be described below, all involve the detection of positron annihilation events in order to ascertain certain information about the material or object being tested.
By way of background, complete annihilation of a positron and an electron occurs when both particles collide and their combined mass is converted into energy in the form of two (and occasionally three) photons (e.g., gamma rays). If the positron and the electron are both at rest at the time of annihilation, the two gamma rays are emitted in exactly opposite directions (e.g., 180° apart) in order to satisfy the requirement that momentum be conserved. Each annihilation gamma ray has an energy of about 511 keV, the rest energies of an electron and a positron.
In positron annihilation analysis, the momentum of the positron is related to the environment in which it resides. For example, positron momentum is relatively low in defects (e.g., microcracks in composite materials and polymers) or in large lattice structures, whereas positron momentum is higher in defect-free or tight lattice structures. One way to determine the momentum of the positron is to measure the degree of broadening of the gamma energy line caused by the annihilation event. Alternatively, the momentum of the positron may be derived from the deviation from 180° of the annihilation gamma rays.
Additional information about the electron density of the material at the site of annihilation may be obtained by determining the average lifetime of the positrons before they are annihilated. Still other information about the annihilation event may be detected and used to derive additional or supplemental information regarding the material being tested, such as the presence of contaminants or pores. Accordingly, the detection of positrons and the products of annihilation events provide much information relating to defects and other characteristics of the material or object being tested.
As mentioned above, several different positron annihilation techniques have been developed. In one type of positron annihilation technique, positrons from a radioactive source (e.g., 22Na, 68Ge, or 58Co) are directed toward the material to be tested. Upon reaching the material, the positrons are rapidly slowed or “thermalized.” That is, the positrons rapidly loose most of their kinetic energy by collisions with ions and free electrons present at or near the surface of the material. After being thermalized, the positrons then annihilate with electrons in the material. During the diffusion process, the positrons are repelled by positively-charged nuclei, thus tend to migrate toward defects such as dislocations in the lattice sites where the distances to positively-charged nuclei are greater. In principle, positrons may be trapped at any type of lattice defect having an attractive electronic potential. Most such lattice defects are so-called “open-volume” defects and include, without limitation, vacancies, vacancy clusters, vacancy-impurity complexes, dislocations, grain boundaries, voids, and interfaces. In composite materials or polymers, such open-volume defects may be pores or microcracks.
Generally speaking, positron annihilation techniques utilizing external positron sources are of limited utility in that the positrons from the external positron sources cannot penetrate very deeply into the materials. As a result, such techniques are limited to evaluating the surface structures of the materials being tested.
Another type of positron annihilation technique replaces the external positron source with an external neutron source. Neutrons from the neutron source are directed toward the material being tested. Given sufficient energies, the neutrons will, in certain materials, result in the formation of isotopes that produce positrons. Such isotopes are commonly referred to as positron emitters, and include certain isotopes of copper, cobalt, and zinc. The positrons produced within the materials by the positron emitters then migrate to lattice defect sites, ultimately annihilating with electrons to produce gamma rays. This type of positron annihilation technique is often referred to as “neutron-activated positron annihilation” because it utilizes neutrons to trigger or induce the production of positrons.
Neutron-activated positron annihilation techniques are advantageous over techniques that utilize external positron sources because the neutrons from the external neutron sources penetrate more deeply into the materials being tested than do positrons alone (e.g., from the external positron sources). Therefore, neutron-activated positron annihilation systems are generally capable of detecting flaws deep within the material rather than merely on the surface. Disadvantageously, however, neutron-activated positron annihilation techniques are limited to use with materials that contain positron emitters (i.e., certain isotopes of copper, cobalt, and zinc).